The disclosed invention generally relates to the fabrication of integrated circuits, and is more particularly directed to a process for maintaining well defined mesa structures and also for maintaining the thickness of oxide over non-active regions while oxide over the active regions is thinned.
In certain known processes for fabricating semiconductor integrated circuits, mesa structures are formed where active devices are to be located. See, for example, "Local Oxidation of Silicon and Its Application in Semiconductor-Device Technology," Appels et al., Philips Res. Repts. 25, 118-132, 1970. After the mesa structures are formed, field oxide is grown and gate oxide is formed. However, in the course of such oxide processing, the mesa structures become narrower in width and less defined as a result of encroachment. Further, gate oxide formation involves etching to thin the oxide, which results in thinning of the field oxide over the no-active regions.
Encroachment and thinning of the field oxide limit the reduction of circuit dimensions. Particularly, mesa structures must be sufficiently wide to compensate for encroachment. Further, thinning of the field oxide over the non-active regions causes increased capacitance, which results in reduced performance. Reduction of circuit dimensions necessarily results in reduced oxide thickness, thereby placing a limit on circuit dimension reduction.